1. Field of the Invention
This invention is concerned with the wood gluing art. It provides an adhesive composition and a method for gluing wood veneers and particles together.
2. Description of Related Art
Many approaches to improving the bonding by wood adhesives have been tried. In particular, a number of workers have tried to improve the interaction between the adhesive and the wood surface, and to increase penetration of the adhesive into the wood.
Penetration is important as shown by Rowell (1978), Int. J. Adhesion Adhesive. 7:183-8, who looked at surface interactions between wood and adhesive by changing the surface properties of the wood. Aspen wood was acetylated with acetic anhydride then glued with phenolic adhesives. The water-soluble phenolic resin could not penetrate into the acetylated wood's surface, and the reduction in penetration resulted in poor bonding.
Lambuth (1967), U.S. Pat. No. 3,342,776, taught a way of increasing the penetration of phenolic resins to increase bond strength in resultant glue bonds. The initial condensation reaction between phenol and formaldehyde is controlled to maximize addition of formaldehyde to phenol while minimizing the dimerization reaction between substituted phenols. The resulting resin has low viscosity to encourage penetration but cures rapidly, and after curing, the glue joint has improved strength.
Hse (1971), Forest Prod. J., 21:44 52; (1972), Holzfors., 26:82-85, studied the effects of variations in surface tension on glue bond strength in southern pine plywood made with phenolic resins. The surface tension was varied by changing the level of sodium hydroxide catalyst present during the condensation reaction of the resin to change the extent of methylol derivatization of the phenol. The glue bond strength, measured as shear resistance or as wood failure percent, increased as surface tension decreased. However, in these experiments the surface tension property was inextricably confounded with the state of condensation in the phenolic resin. Therefore, it is unclear whether the improved bond strength is really the result of changes in surface tension or whether the true effector is the change in conditions during the condensation reaction. Perhaps as a result, other components to modify surface tension such as surfactants have not been tried with phenolic resins for southern pine plywood.
Eblez (1978), Holzfors., 32:82-92, reported studies with resorcinol adhesives for beech plywood, where the surface tension was modified by addition of low molecular weight aliphatic alcohols. Some improvement in strength was observed on the addition of methanol or isopropanol, although the concentration is unspecified. Addition of benzyl alcohol had no effect on the glue strength. These changes in glue strength seemed to correlate inversely with the wetting ability of the adhesive. In general, as wetting ability of the adhesive mixture increased, the strength of the resulting bond decreased, which argues against using surfactants to change adhesive wetting ability.
Modification of urea-formaldehyde adhesives with surfactants has been attempted, but has not been uniformly successful. Horioka, et al. (1956) Jap. Forest Exp. Stn. Bull., 89:1-55, reported that a small amount of a surface active compound had little or no effect on the bond strength of beech veneer plywood made with urea-formaldehyde adhesive. When plywood panels were soaked before they were tested, presence of a surface active compound in the adhesive decreased the bond strength. When plywood was not pre-soaked before testing, a small amount of surfactant in the adhesive gave some increase in bond strength, while increasing amounts of surfactant lowered bond strength.
Herczeg (1965), Forest Prod. J., 15:499-505, used polyoxyethylene sorbitan esters to modify the surface tension of urea adhesives. When these adhesives were used to make plywood, he found the highest bond strength for resins containing 0.025% polyoxyethylene sorbitan esters.
The art of plywood manufacture has long recognized that manufacture of plywood from wet veneers results in poor bonding of the finished products, and similar effects have been observed in other composite wood products manufactured from wood furnish whose moisture content was too high. Adhesives formulated to provide good bonding for veneers with low moisture content (from 0-6%), give poor results when applied to intermediate or high moisture veneer (7-25%), and an adhesive formulated for use with intermediate moisture veneer will not run properly on plywood forming machines set to accommodate low moisture veneers. Wet furnish or veneer tends to cause over-penetration during hot-pressing, so that too little active bonding component is retained at the interface after curing is complete. The excessive flow is caused both by dilution of the adhesive and by retardation of cure, which produces a longer flow period. If particulate extenders are present, they cannot perform their usual function of holding active bonding component such as resin at the interface, and instead become detrimental by further weakening the cured interfacial layer. Many different solutions to the problem of over-penetration have been proposed including both adding modifiers to the adhesive and changing the way the adhesive resins are prepared in order to change the penetration properties of the adhesive.
Kuhr (1967), Canadian Patent No. 754,646, noted when gluing plywood with phenolic resins that the efficacy of bonding is sensitive to the moisture content of the wood and also that using too high temperature during hot press curing of plywood made with high-moisture-content veneer often resulted in steam blisters, damaging the panels. Kuhr taught that the addition of between 0.75% and 4.5% polyvinyl alcohol resin to phenol-formaldehyde adhesive when manufacturing either plywood or chipboard improves stability of the adhesive before pressing, and permits lower press temperatures as well as reduced drying time between applying the adhesive and pressing. Inclusion of polyvinyl alcohol is taught to reduce the sensitivity of the adhesive to variation in initial wood moisture content and to reduce the frequency of steam blisters as a defect.
Most of the techniques available for minimizing over-penetration of adhesive merely shift the emphasis to another manufacturing problem-type. For instance, using higher molecular weight resin will reduce over-penetration but at the expense of reducing the amount of time available between application of the resin and final curing of the glue bond. Addition of a limited amount of vinyl resin to phenolic adhesives, as reported by Kuhr, has relatively few detrimental side effects, so long as the amount of vinyl resin added is small, but as a result the benefit is also limited. There exists a need for adhesive modifiers which reduce over-penetration and which may serve as an alternative to vinyl resins or may be used in an additive fashion with the existing modifiers.